Thermoeconomic analysis and optimisation of biomass fuel gas turbines
The ready availability of biomass in Brazil makes this type of fuel a major candidate to integrate the country's energy matrix. Although this fuel is used as a primary energy source, its use for electricity generation is still modest. On the other hand, high efficiency and power density achieved by modem gas turbine engines make them a promising option for the power generation market. Thus, this thesis has as main objective to analyse the marriage between the solid fuel, biomass in this case, and gas turbines. Two main types of power plants are studied; the biomass integrated gasification gas turbine cycle (BIGGT) and the externally fired cycle (EFGT), which for the first time is thoroughly studied for the use of biomass fuel, plus the intercooled and recuperated variants of these power plants. The results are compared with the ordinary natural gas fuelled cycle. The method involves on- and off-design point performance and exergy analysis. The economic performance and optimisation for each cycle is also explored in order to assess their feasibility. The optimisation technique adopted is the Genetic Algorithm (GA) connected to the conventional hill-climbing methodology. This merge uses the GA to identify the region of optimum values, which are then passed on to the hill-climbing algorithm. In this way the long time demanded by the GA to converge is shortened and the unreliability of the hill-climbing method in finding the global optimum is overcome. The codes developed for design-point performance analysis and optimisation, compared with a commercial package, proved reliable and robust. The tools developed for exergy analysis (on- and off-design) are also robust and flexible, with the capability of analysing and calculating the properties of mixtures made of 23 different gases. The emissions equations are sufficiently accurate for the purposes of this thesis. The relationship proposed for calculating the variable operating and maintenance costs proved to be consistent with the current knowledge. The results show that the optimised cycles are competitive with current technology in terms of cost of electricity, the EFGT being the more competitive biomass cycle, with costs of electricity (US$ 0.07/kWh) comparable with those of the natural gas fuelled power plants. The BIGGT in its turn shows a cost of electricity 29 percent higher than its natural gas and externally fired counterparts (US$0.09/kWh) counterparts. The method used to work out the best investment - the required revenue (RR) method - demonstrated that the EFGT is again comparable with the NGGT cycle, with its RR being only 7 percent higher. The BIGGT cycle shows a higher RR due to its costly gasification/cleaning system. The minimisation of the exergy destruction ratio indicates that little improvement would be achieved after the reduction of this parameter, and a penalty - an 85 percent increase in the cost of electricity - must be paid. The environmental advantage of the biomass-fuelled cycles over the natural gas cycle is clear, making these systems very promising as low emissions alternatives. Both BIGGT and EFGT cycles presented very low CQ2 emissions. Regarding NO., emissions, the EFGT cycle has the lowest rates, whereas the BIGGT has the highest.